Generally, vibratory gyroscopes are known as devices for measuring the angular velocity of a rotating object using the fact that a Coriolis force perpendicular to vibration and an angular velocity vector of the rotating object acts on an object vibrating on the rotating object, and have been used as devices for position confirmation in airplanes, large ships, space satellites, and so on. Recently, in the field of consumer products, vibratory gyroscopes are used for position measurement in car navigation, car posture control, detection of VTR-camera or still-camera hand movement, and so on.
There have conventionally been known vibratory gyroscopes of various structures comprising a vibrator having a body portion and a pair of drive arms and a pair of detection arms connected to the body portion, wherein both pairs extend in mutually opposite directions in the same plane from the body portion. The vibrators are mainly manufactured by anisotropic etching of Z-cut quartz.
As a conventional vibratory gyroscope, there is one disclosed, for example, in Japanese Unexamined Patent Application Publication (JP-A) No. H11-14373 (Document 1).
FIG. 1 is a basic structural diagram showing one example of a vibrator for use in a vibratory gyroscope. Technical explanation thereof is given in detail in the prior art of Document 1. An illustrated vibrator 110 comprises a rectangular flat plate body portion 111, a pair of mutually parallel drive arms 112 and 113 connected to the drive side of the body portion 111, and a pair of mutually parallel detection arms 114 and 115 connected to the detection side of the body portion 111. The vibrator 110 is formed into a flat plate shape in the XY plane and has a thickness in the Z-direction. Further, the vibrator 110 has a symmetrical axis which is a central axis thereof along its longitudinal direction.
The drive arms 112 and 113 are driven by non-illustrated drive means so as to cause vibrations in mutually opposite phases in the XY plane. The vibrations in the XY plane are not transmitted to the detection arms 114 and 115 through the body portion 111. In this state, if a rotational angular velocity Ω is exerted about the symmetrical axis of the vibrator 110, Coriolis forces act on the drive arms 112 and 113. Because of the drive arms 112 and 113 being vibrating, vibrations in mutually opposite phases in the YZ plane are induced to the drive arms 112 and 113. Since the shape is determined so that the drive arms 112 and 113 and the detection arms 114 and 115 resonate together in the YZ plane, the vibrations of the drive arms 112 and 113 in the YZ plane caused by the Coriolis forces are induced to the detection arms 114 and 115. The induced vibrations are detected by non-illustrated detection means provided at the detection arms 114 and 115 and are used for measuring the rotational angular velocity.
Normally, for resonance, the length dimension of each drive arm 112, 113 provided in one direction of the body portion 111 is designed to be equal to that of each detection arm 114, 115. However, for achieving the weight balance by matching a geometric center position 121 of the body portion with a position of the center of gravity of the entire vibrator 110 in order to stabilize the vibrations, the width dimension of each drive arm 112, 113 and that of each detection arm 114, 115 should be set equal to each other. In this state, the symmetrical drive mode of the drive arms 112 and 113 in the XY plane indicated by arrows H in the figure is most susceptible to propagate to the detection arms 114 and 115. This is because resonant frequencies of the respective drive arms 112 and 113 and detection arms 114 and 115 in the direction in the XY plane become equal to each other.
However, with such a configuration, vibrations that would otherwise be produced by the detection arms 114 and 115 in the direction in the YZ plane perpendicular to the XY plane indicated by arrows V in the figure are significantly impeded due to such a resonant frequency. Accordingly, there is a drawback of causing a reduction in S/N ratio and gyro resolution.
As a measure of improving this, Document 1 discloses a plurality of vibratory gyroscopes. One of those vibratory gyroscopes comprises a vibrator having three drive arms and two detection arms. This will be described with reference to FIG. 2.
As illustrated, three drive arms 132, 133, and 134 extending on one side of a rectangular flat plate body portion 131 are arms for vibration excitation in the XY plane that are all excited by non-illustrated drive means through electrodes 137, 138, and 139. Phases of vibrations of the adjacent drive arms are opposite to each other. In this case, an angular velocity Ω is exerted about a longitudinal axis of this vibrator 130, so that the adjacent drive-side arms receive mutually oppositely directed Coriolis forces in the direction perpendicular to the XY plane. The Coriolis forces are transmitted to detection arms 135 and 136 through the body portion 131 and detected by non-illustrated detection means connected to electrodes 140 and 141, so as to be used for measuring the rotational angular velocity.
However, there is a drawback that since the Coriolis force applied to the middle drive arm 134 is in the opposite direction as compared with those applied to the drive arms 132 and 133 at both ends, the Coriolis forces transmitted to the detection arms 135 and 136 through the body portion 131 are largely reduced and thus the gyro sensitivity is deteriorated. That is, as a result of making such a configuration on an experimental basis and evaluating it, it was only possible to obtain a low-sensitivity piezoelectric gyroscope.